In high frequency communication systems typified by mobile phones, high-frequency filters are used to remove unnecessary signals other than signals in the frequency band used for communication. Acoustic wave resonators such as surface acoustic wave (SAW) resonators are used in the high-frequency filters. In the surface acoustic wave resonator, an Interdigital Transducer (IDT) having a pair of comb-shaped electrodes is formed on a piezoelectric substrate such as a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate.
In the acoustic wave resonator, desired input and output impedance can be set by setting the electrostatic capacitance between a pair of comb-shaped electrodes as disclosed in, for example, Japanese Patent Application Publication No. 2004-146861 (hereinafter, referred to as Patent Document 1). The electrostatic capacitance of the pair of comb-shaped electrodes can be set by changing the aperture length and the number of pairs. It has been known to reduce spurious by making the acoustic velocity of the acoustic wave in the edge region of the overlap region of the pair of comb-shaped electrodes lower than the acoustic velocity of the acoustic wave in the center region of the overlap region as disclosed in, for example, Japanese Patent Application Publication Nos. 2011-101350 and 2017-112603 (hereinafter, referred to as Patent Documents 2 and 3, respectively).